1. Technical Field
The present invention relates generally to vehicle driving control apparatuses. More particularly, the invention relates to a vehicle driving control apparatus that controls an own vehicle to track (or follow) a preceding vehicle running ahead of the own vehicle.
2. Description of Related Art
To reduce the operating burden of a driver driving a vehicle, there have been developed vehicle driving control apparatuses that select a preceding vehicle, which runs ahead of the own vehicle, as a target vehicle, and control the speed of the own vehicle so as to keep the inter-vehicle distance between the own vehicle and the target vehicle constant, thereby making the own vehicle automatically track (or follow) the target vehicle.
For example, there is disclosed a vehicle driving control apparatus (or inter-vehicle distance control apparatus) in Japanese Patent Application Publication No. JP2010143323A. The vehicle driving control apparatus is configured to perform a tracking control, i.e., a control of making an own vehicle track a target vehicle (or preceding vehicle). Moreover, the apparatus is further configured to: stop the tracking control upon start of an accelerating or decelerating operation by a driver of the own vehicle; reset (or change) a target inter-vehicle distance based on the actual inter-vehicle distance when the relative speed between the target vehicle and the own vehicle becomes 0 after completion of the accelerating or decelerating operation; and resume the tracking control with the reset target inter-vehicle distance. Consequently, when the tracking control is resumed, it is unnecessary to perform additional deceleration and acceleration of the own vehicle for maintaining the target inter-vehicle distance.
However, besides the completion of an accelerating or decelerating operation made by the driver during the tracking control, there may also occur the following events where the relative speed between the target vehicle and the own vehicle changes not gradually but discontinuously: (a) an intervening vehicle, which intervenes between the own vehicle and the target vehicle, becomes a new target vehicle to track; and (b) the driver initiates the tracking control by operating a switch. In addition, the relative speed is positive when the target vehicle runs at a higher speed than and thus gets further from the own vehicle and negative when the target vehicle runs at a lower speed than and thus gets closer to the own vehicle.
In a normal tracking control, when the relative speed between the target vehicle and the own vehicle discontinuously changes, the own vehicle will be suddenly accelerated or decelerated so as to make the own vehicle track the target vehicle. Consequently, a shock will be caused by the sudden acceleration or deceleration of the own vehicle, thereby lowering the driveability of the own vehicle. In addition, this shock will be shortly referred to as acceleration shock or deceleration shock hereinafter.
The aforementioned patent document fails to disclose how to reduce the acceleration shock or deceleration shock caused by the tracking control when the relative speed between the target vehicle and the own vehicle discontinuously changes.